The method of heat softening a glass article of prescribed weight, known as a preform, and press molding it in a pressing mold is widely employed to efficiently manufacture optical glass elements such as aspherical lenses. In this method, an optically functional surface such as a lens surface is precisely molded by press molding, obviating the need for mechanical processing such as grinding or polishing of the optically functional surface. Normally, the above method is called precision press molding or mold optics molding.
In precision press molding, high internal quality and surface quality are required of the preform. When the internal quality is low, only optical elements of low internal quality can be obtained. Further, the surface quality of molded products obtained using preforms of low surface quality is also poor. Since the optically functional surface of an optical element manufactured by precision press molding is not subjected to mechanical processing, only optical elements with low quality optically functional surfaces can be obtained.
There are also methods of manufacturing the above preforms in which a preform of prescribed weight is obtained by mechanically processing a glass material and in which a prescribed weight of glass melt is molded to obtain a preform. The latter method is referred to as hot preform forming; this is a good method for mass-producing high-quality preforms. An example of such a hot preform forming method is disclosed in Unexamined Japanese Patent Publication (KOKAI) Heisei No. 8-81228 (Reference 1).
In the method described in Reference 1, multiple dies positioned on an index table are rotated and a single die is displaced vertically at a casting position to receive a prescribed weight of glass melt on the die. When being displaced, the single die moves directly beneath a nozzle, and when the die is raised and stopped, glass melt is loaded from the nozzle onto the forming surface. When the die is lowered at a timing yielding the required weight, the glass melt is pulled away from the nozzle. A prescribed weight of glass is then present on the die. The particular die is then moved out from under the nozzle and the next die moves beneath the nozzle. This is continuously repeated to cut the glass and manufacture glass gobs.
Since cutting is not conducted based on shear, this method is well suited to manufacturing high-quality glass elements without defects knows as shear marks.
However, since the cutting operation and the die rotating operation are conducted by a single mechanism, increasing the number of preforms produced requires shortening the time during which the next die moves under the nozzle after cutting the glass. It becomes necessary to increase the rate of movement of the table, increasing the acceleration in a horizontal direction (horizontal acceleration) that is exerted on the glass melt during movement. Since the glass supplied to the die remains briefly in a state of elevated temperature after being fed to the die, the exertion of a large horizontal acceleration causes defects such as deformation. Further, a high horizontal acceleration may also cause problems such as the generation of gaps in the surface of the preform. That is, so long as the cutting operation and the die rotating operation are conducted by a single mechanism, the increase in the number of units produced without the above-stated problems occurring is spontaneously limited.
Further, since the height of the die during cutting affects the weight precision of the preform, manufacturing preforms with precise weight precision requires microadjustment of the height of multiple dies. However, microadjustment of the height of multiple dies constitutes a substantial burden.
The first object of the present invention is to provide a more rapid method of manufacturing higher quality glass articles which overcomes the drawbacks of conventional glass article manufacturing methods, and a method of manufacturing optical elements employing the preforms manufactured by this method.
Methods of manufacturing preforms for press molding (press-molding preforms), in which the glass melt flows out of a nozzle, is received by a die, and is formed, are known. An example of such methods is disclosed in Unexamined Japanese Patent Publication (KOKAI) Heisei No. 8-81228 (Reference 1). In this method, the front portion of the glass melt that is about to drip from a nozzle is supported, and at a timing designed to separate a prescribed weight of glass, the receiving die is dropped more rapidly than the flow rate of the glass melt. The front end portion of the glass melt, still supported by the receiving die, separates from the nozzle, yielding glass melt of prescribed weight on the receiving die. Subsequently, the glass melt is simultaneously cooled and formed into a preform. In this method, it is possible to adjust the distance between the receiving mold and the front end of the nozzle to vary the weight of the preform.
In addition to the method disclosed in Reference 1, there is a forming method in which a glass droplet naturally dripping from a nozzle is received and formed by a receiving die.
In the method described in Reference 1, since flowing glass is successively received by multiple receiving dies, precisely regulating the weight of the preforms to a prescribed weight requires precise adjustment of the height of each receiving die as it receives the glass. Further, the glass on the receiving die loses heat from the surface of the glass to a die with which it comes into contact, while the upper portion consists of hot glass that has been newly fed by the nozzle. Thus, the viscosity within a single glass gob is rendered nonuniform. For this reason, there is a problem in that well-shaped preforms cannot be obtained from glass undergoing considerable change in viscosity with change in temperature (known as “short” glass). There is also a problem in that striae tend to form due to convection within the glass.
In the method of forming glass droplets of naturally dripping glass, it is possible to control the weight of the glass droplets more precisely than in the method described in Reference 1 by keeping the glass flow conditions constant. However, the timing at which the glass drips down is determined by a balance between surface tension and the gravitational force exerted on the glass at the front end of the nozzle. Specifically, denoting the diameter of the glass melt in droplet form at the front end of the nozzle as “D”, a parameter representing the magnitude of the surface tension as “γ”, gravitational acceleration as “g”, and the mass of the glass when dripping as “M”, this timing is approximately determined by Mg=γπD. Accordingly, it is difficult to manufacture a preform of a weight where the surface tension is less than the above gravitational force. There is thus a drawback in the form of a low degree of freedom in setting the weight.
Based on these problems, there is the need for a technique of forming glass preforms of good shape and high quality that both affords a high degree of freedom in setting the weight of the preform and achieves molding of high precision with respect to the weight that is set.
The second object of the present invention is to provide a method of manufacturing glass gobs affording good shape, high quality, free choice of weight, and high weight precision in response to the above-stated need, as well as a method of manufacturing optical elements by press molding the glass gobs produced by the latter method.